Why Unit Conversions Matter

Unit conversions trip people up in dozens of everyday situations: following an American recipe, understanding your weight when a doctor uses different units, converting miles to kilometres when driving abroad, or working out whether 40 degrees Celsius is warm or dangerously hot. Most unit conversions come down to a small set of multiplication or division factors — once you know them, conversion becomes mechanical.

Length Conversions

The metric system (metres, centimetres, kilometres) is used across most of the world. The imperial system (inches, feet, miles) remains in everyday UK contexts — road distances, heights, screen sizes.

  • 1 inch = 2.54 centimetres (exact, by international definition)
  • 1 foot = 30.48 cm = 12 inches
  • 1 metre = approximately 39.37 inches = approximately 3.28 feet
  • 1 mile = 1.609 kilometres
  • 1 kilometre = 0.621 miles

Shortcuts: Km to miles: multiply by 0.6. Miles to km: multiply by 1.6. A person 5ft 10in (70 inches) tall is 70 × 2.54 = 177.8cm.

Weight and Mass Conversions

In the UK, both kilograms and the stone/pound system remain in common use for personal body weight.

  • 1 pound (lb) = 453.6 grams = 0.4536 kg
  • 1 kilogram = 2.205 pounds
  • 1 stone = 14 pounds = 6.35 kilograms
  • 1 ounce (oz) = 28.35 grams
  • 1 metric tonne = 1,000 kilograms = 2,205 pounds

Shortcuts: Kg to pounds: multiply by 2.2. Kg to stones: divide by 6.35. In cooking: 100g ≈ 3.5 oz; 500g ≈ 1lb 2oz.

Temperature Conversions

Temperature conversions require both multiplication and addition/subtraction due to different zero points:

  • Celsius to Fahrenheit: F = (C × 9/5) + 32
  • Fahrenheit to Celsius: C = (F − 32) × 5/9
  • Celsius to Kelvin: K = C + 273.15

Key reference points: 0°C = 32°F (water freezes); 20°C = 68°F (comfortable room); 37°C = 98.6°F (body temperature); 100°C = 212°F (water boils); −40°C = −40°F (only point where both scales are equal).

Quick mental maths: Double the Celsius figure and add 30 for a rough Fahrenheit equivalent. 20°C → (20×2)+30 = 70°F (actual: 68°F). Useful when you just need a sense of whether to pack a coat.

Volume Conversions

UK and US use different definitions of the same unit names — be careful when following recipes from either country.

  • 1 litre = 1,000 millilitres
  • 1 UK pint = 568 ml; 1 UK gallon = 4.546 litres
  • 1 US pint = 473 ml (smaller than UK); 1 US gallon = 3.785 litres
  • 1 US cup = 240 ml; 1 US tablespoon = 15 ml; 1 US teaspoon = 5 ml

When following American recipes, cup measurements are consistent (1 cup = 240ml), but if a recipe says "1 pint," it means 473ml (US), not 568ml (UK).

Area Conversions

Used in property (square footage vs square metres), land measurement, and flooring projects.

  • 1 square metre = 10.764 square feet
  • 1 square foot = 0.0929 square metres
  • 1 acre = 4,047 square metres = 0.405 hectares
  • 1 hectare = 10,000 square metres = 2.47 acres

Property shortcut: To convert square metres to square feet, multiply by 10.8. A 75m² flat is approximately 807 square feet.

Speed Conversions

  • 1 mph = 1.609 km/h
  • 1 km/h = 0.621 mph
  • UK motorway limit 70 mph = approximately 113 km/h
  • EU motorway limit 130 km/h ≈ 81 mph

Digital Storage Conversions

Data storage uses powers of two (binary), which creates a discrepancy with how manufacturers label storage devices:

  • 1 kilobyte (KB) = 1,024 bytes (binary) or 1,000 bytes (decimal)
  • 1 megabyte (MB) = 1,024 KB; 1 gigabyte (GB) = 1,024 MB
  • 1 terabyte (TB) = 1,024 GB

This is why a "1TB" hard drive shows approximately 931GB when formatted — manufacturers count in decimal while operating systems count in binary.

Cooking Measurement Conversions

Converting between weight and volume in cooking depends on the ingredient's density:

  • 1 cup water = 240ml = 240g
  • 1 cup plain flour = approximately 120g to 130g
  • 1 cup granulated sugar = approximately 200g
  • 1 cup butter = approximately 227g (2 sticks in US measurements)

For baking especially, using weight rather than volume produces more consistent results — a cup of flour can vary by up to 40% in weight depending on how it is scooped.

A Note on Precision

Many conversion factors are defined exactly (1 inch = 2.54cm by international agreement), while others are approximations. For everyday use, approximations are sufficient. For scientific, engineering, or legal purposes, always use the precise definition and account for significant figures.

Conclusion

The key factors to memorise are: 1 inch = 2.54cm, 1 mile = 1.609km, 1kg = 2.205lb, and the temperature formulas. For everything else, use our free Unit Converter for instant, accurate conversions across length, weight, temperature, volume, speed, and more.